The invention relates generally to color copying.
More particularly, the invention relates to a method and an arrangement for automatic exposure control in color copiers, especially photographic color copiers.
The so-called neutral gray correction is employed with advantage when making copies of a colored original, that is, a color negative or positive, having sharp color contrasts. When using the neutral gray correction, the exposures in the individual colors are controlled in such a manner that the copy as a whole has a neutral gray density. The neutral gray correction is based on the recognition that better results are obtained when originals having sharp color contrasts are exposed so as to yield neutral gray copies than when these are exposed so as to reproduce the color variations of the originals on the copies. However, as the materials for the originals continue to improve, color contrasts become less and less pronounced. As a result, the so-called dominants due to the object imaged onto the original are suppressed when the neutral gray correction is used so that unsatisfactory reproductions of the colors are achieved. The reproductions of the colors are improved by means of an undercorrection.
In view of the above, most modern color copiers are designed for color undercorrection. A predetermined percentage of the full undercorrection factor is used and, although adjustable, generally remains unchanged during the processing of a series of originals. A color copier of this type is disclosed, for example, in the U.S. Pat. No. 3,867,029.
Neutral gray correction and undercorrection are mutually exclusive in that one increases as the other decreases. Thus, in establishing the percentage of the undercorrection factor to be used, it is necessary to compromise between the color contrasts which it is desirable to eliminate by means of the neutral gray correction and the color dominants which it is desirable to retain to the extent possible, that is, which are to be copied with a high degree of undercorrection. This compromise regularly causes difficulties, particularly when a roll containing sections of film which have different color compositions or which must be pretreated differently are to be processed in an automatic color copier.